Quick and harmless fix to help with alternate OpenSearch schemas

[tloubrieu-jpl]

Related Issue(s):

• Partly helps with Allow injection of custom Elasticsearch/OpenSearch mappings via configuration #544
  (I did not want to create a new ticket which could be redundant with this one)

Description:

Have opensearch datetime, geometry and collections fields defined as constant string so to be easily overwritten when the OpenSearch mapping requires it, as shown in example https://github.com/NASA-PDS/registry-stac-api/blob/464ce37952367fa845b325e36263b04a1fd89965/src/pds/registry/stac/database_logic.py#L28

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable
• Changes are added to the changelog

[jonhealy1]

Hi @tloubrieu-jpl This looks really good. Can you add the same changes to the Elasticsearch backend. Thanks!